1. Field of the Invention
The present invention relates to technology in which multiple types of digital information, such as video and audio, are multiplexed and transmitted/received between media devices connected to a network. More specifically, the present invention relates to a multiplexing network system and a digital-information transferring method which use a specific bus that allows synchronous transfer in real time, for example, a multiplexed-data bus based on IEEE 1394 (hereinafter, the bus will also be simply referred to as a “1394 bus”).
2. Description of the Related Art
In a multiplexing network system, a specification (a bandwidth) thereof restricts the number and/or data sizes of streams multiplexed and transferred. Thus, in general, a new device that exceeds the number of media devices connected, the number being contemplated during design, cannot be added to the multiplexing network system (i.e., the system cannot be expanded). That is, even if an attempt is made to add a media device that was not intended in the system design, the addition is not easy (the addition is practically impossible). This is because, at the stage of system design, a media device or media devices that would be added in the future are assumed in addition to originally connected media devices, the compression rates of streams for the media devices are set to have fixed values allowing for the media device(s) to be added and a band on the network is allotted thereto.
In order to add a media device intended during the design, a band is assigned to the media device with a compression rate set during the design, based on an instruction from a master device connected to the network, and when the device is actually connected, video/audio data is transmitted in the band pre-assigned during the design. On the other hand, a sink device, such as a display, connected to the network receives the video/audio data in that band on the network and performs playback (such as screen display and sound output), based on an instruction from the master device.
Such a multiplexing network system is also incorporated into vehicles, and is realized as, for example, infotainment systems. The term “infotainment” is a combination of “information” that refers to information (e.g., navigation information) required for the vehicles and “entertainment” that refers to entertainment information (e.g., video/audio information of various sources) offered to the drivers and passengers (particularly users in the rear seats). Currently, OEMs (original equipment manufacturers) have strong demands for multi HMIs (human machine interfaces) in a next-generation vehicle-mounted infotainment system, thus increasing a need for a multiplexed-data bus, such as a 1394 bus.
A recent trend is that, for example, video data size (the amount of information) is increasingly becoming large and digitization of an entire system is progressing. In view of such a situation, it is necessary to provide a multiplexing network system with more flexibility.
As technologies related to the above-described known technology, for example, there is a technology in which a significance for each type of video, audio, and so on or a significance for each object that appears in video, audio, and so on is determined and a relatively large amount of information is assigned to a higher significance to perform multiplexing, as is disclosed in Japanese Unexamined Patent Application Publication No. 2002-57638. Also, as is disclosed in Japanese Unexamined Patent Application Publication No. 2004-104529, for a vehicle-mounted network system in which multiple networks provided in a vehicle are interconnected by a media converter apparatus, there is a technology in which the media converter apparatus constantly detects an available bandwidth in the network to which data is to be output, compresses and converts image data into a compression format that can be output in the detected available bandwidth, and outputs the compressed and converted data to the network. In addition, as disclosed in Japanese Unexamined Patent Application Publication No. 2004-241885, for an information transfer system in which multiple types of digital information are multiplexed and synchronously transferred between multiple processing devices connected to a network, there is a technology that allows, when a bandwidth required for the synchronous transfer is not ensured in the network, the bandwidth to be assigned in the range of a currently available bandwidth to start synchronous transfer.
As described above, in the known technologies, when a multiplexing network system is configured, a media device that is not intended during system design cannot be added later, because of the restriction on the availability of the band. Thus, if the unintended media device is added to the system, a band that is specified by a specification of the system is exceeded, and consequently, not only video/audio data of the added device but also video/audio data of other preinstalled media devices cannot be transferred in that band.
That is, the known technologies have a problem in that a new media device that exceeds the number of media devices, the number being set at the system design stage, cannot be added to an existing multiplexing network system to expand the system. The in-band transfer becoming impossible due to exceeding the band is not preferable in terms of efficient use of the band, and also leads to impairment of flexibility of the system configuration.
Problems of the known technologies will now be described with reference to FIG. 6.
FIG. 6 is a diagram schematically illustrating an example of use of a band (an isochronous area) on a 1394 bus. The 1394 bus uses a maximum of 100 μs (an isochronous area) of one transfer cycle (125 μs), as illustrated, to transfer streams. In the illustrated example, four streams of four sources A, B, C, and D occupy the period of 100 μs. In order to add a stream (i.e., to add a new media device) to such a multiplexing network system in which bands are assigned to respective media devices (e.g., the sources A, B, C, and D), it is necessary to reduce the sizes of currently transferred streams and to ensure a band for the new stream.
While reducing the stream sizes requires the use of a compression technology, the compression rates of streams for media devices intended during the system design are set to have fixed values in the known technologies, as described above. Therefore, even when an attempt is made to later add a media device not intended during the design, a bandwidth that is appropriate for a band for the added media device cannot be ensured and the band is consequently exceeded. As a result, there are problems in that a band cannot be efficiently used, flexibility of the system configuration is impaired, and the system cannot be expanded.